The burning of fossil fuels, including coal, is necessary to meet the energy requirements of our society. However, the combustion of coal, and in particular, many lower grades of coal emits sulfur oxides into the atmosphere. Additionally, nitrogen oxides are produced during combustion. Some nitrogen oxides are derived from fuel-bound nitrogen, while some are derived from atmospheric nitrogen. High flame temperatures fix the nitrogen in combustion gases to one or more oxides of nitrogen. Single-stage burners are associated with high flame temperatures. One method of reducing the formation of nitrogen oxides is to employ burners in which combustion is staged to lower flame temperatures.
The release of sulfur and nitrogen compounds produces many detrimental environmental effects. Respiration of these pollutants can cause human health problems ranging from mild respiratory irritation to more serious chronic diseases. Both sulfur oxides and nitrogen oxides can also react with other compositions in the atmosphere to form acid precipitation which has the effect of acidifying bodies of water and destroying the wildlife which live in such habitats. Acid precipitation can also destroy man-made structures such as buildings and statues.
Industry has sought to burn low sulfur coal to avoid problems associated with sulfur oxide emissions. However, such fuel is not always readily available and the costs to recover and transport such high quality coal is in many cases prohibitive. Therefore, to meet the objective of environmentally acceptable coal combustion, methods have been developed to remove sulfur compounds from the coal before combustion and to remove sulfur and nitrogen oxides during the combustion process as the gases are being cooled, but before release to the atmosphere.
Recent revisions in the Federal Clean Air Act applicable to new sources require, for high sulfur coal, a ninety percent reduction in pounds of sulfur per million Btu before release to the atmosphere of combustion by-products. The Clean Air Act, therefore, makes it necessary to achieve higher reductions in sulfur in emissions from the combustion of coal.
Numerous methods for removing sulfur oxides from gaseous waste streams are known, including wet scrubbing processes and sorbent sulfur capture. The primary goal of such methods is to cause a chemical reaction between sulfur oxides and some additive to form a compound which can be recovered prior to releasing the waste combustion gas stream to the atmosphere. In wet-scrubbing processes, waste gas is passed through a slurry containing a calcium or magnesium compound. The sulfur compound in the waste stream reacts with the calcium or magnesium compound to form an insoluble compound which is effectively removed from the waste gas stream. For example, SO.sub.2 dissolves in water to form H.sub.2 SO.sub.3 which reacts with hydrated lime (Ca(OH).sub.2) to form insoluble calcium sulfite. Wet scrubbing techniques, however, are expensive, require retro-fitting, and can be easily fouled by precipitation or insoluble calcium salts inside the scrubber. Additionally, if a wet scrubbing unit is shut down for maintenance, the power plant must frequently be shut down, as well.
Sulfur compounds can also be captured from a waste combustion gas stream by introducing a material containing an alkaline earth metal, commonly calcium, as a sorbent to the combustion system. In such processes, an alkaline earth metal oxide is formed during combustion and reacts with sulfur oxides to form solid sulfur containing compounds which can be removed from the exhaust gas with, for example, electrostatic precipitators. The reactions by which sulfur is captured involve a series of complex physical and chemical processes which are not completely understood. The sulfur-capture reactions involving limestone are believed to involve the following calcination and sulfation reactions: EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2 ( 1) EQU CaO+SO.sub.2 +1/2O.sub.2 .fwdarw.CaSO.sub.4 ( 2)
Calcium sulfate (CaSO.sub.4) is a solid material which can be removed from the exhaust gas before release to the atmosphere.
The use of calcium based sulfur sorbents is well known. For example, in Maloney, Sulfur Capture in Coal Flames, AIChE Symposium Series, Vol. 76, (1980) methods of sulfur capture by the use of alkaline earth metals added to the combustion chamber of coal fired boilers are reviewed. Maloney states that sulfur retention levels off at about eighty-five percent at calcium to sulfur (Ca/S) molar ratios of 3.5 to 4.
Giammar, et al., Evaluation of Emissions and Control Technology for Industrial Stoker Boilers EPA 600/7-81-090, p. III-86 (May 1981) conducted studies on pellets formed from limestone and coal with Ca/S molar ratios of 3.5, and obtained sulfur retention of up to 67%. With a Ca/S molar ratio of 4, sulfur retention of 64% was attained. As the Ca/S molar ratio was increased to 7, sulfur retention increased to 73%.
Zallen, et al., The Generalization of Low Emission Coal Burner Technology, Proceedings of the Third Stationary Source Combustion Symposium, Vol. 2, EPA 600-7, 79-050B, February 1979, disclosed a system where limestone is pulverized with coal and directly fired in a low NO.sub.x burner boiler simulator. For Ca/S molar ratios of 1, 2, and 3, sulfur captures of 50%, 73%, and 88%, respectively, were achieved. In all three tests, the coal used was Utah low sulfur coal.
Liang, et al., Potential Applications of Furnace Limestone Injection for SO.sub.2 Abatement, presented to Coal Technology Conference, Houston, Tex., Nov. 13-15, 1984, compared the effectiveness of SO.sub.2 reduction between injection of limestone at various locations in coal boilers. Liang reported sulfur dioxide capture of about 34% for limestone mixed with coal prior to combustion, about 38% for limestone introduced between burners, and about 51% for limestone introduced into the upper furnace. In these experiments, Ohio #6 coal was combusted and Kemco limestone was the source of calcium. Liang concluded that limestone injection with the coal is the least effective method for sulfur dioxide capture and that injection through the upper furnace ports achieves the highest capture levels for a given set of furnace conditions.
In one study, Cole, et al., Reactivity of Calcium-Based Sorbents for SO.sub.2 Control, Proceedings: First Joint Symposium on Dry SO.sub.2 and Simultaneous S.sub.2 /NO.sub.x Control Technologies, EPA-600/9-85-020a, Paper No. 10 (July 1985), sulfur sorbent reactivity was compared. In terms of calcium utilization, dolomites were most reactive, hydroxides were less reactive, and calcites were least reactive, based on the percent calcium from the sorbent as a sulfate.
A well recognized problem associated with introduction of sulfur sorbents into combustion zones is sintering of sorbents due to high temperatures which causes loss of sulfur capture capacity. Martin, et al., EPA's LIMB R&D Program-Evolution, Status, and Plans, Proceedings: First Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies, EPA-600/9-85-020a, Paper No. 3 (July 1985), recognized the sintering problem, also known as "dead burning", as the heating of limestone to a temperature above which fresh calcium oxide recrystallizes, causing the sulfur capture reactivity to decrease dramatically due to a loss of surface area.
Rakes, et al., Performance of Sorbents With and Without Additives, Injected Into a Small Innovative Furnace, Proceedings: First Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies, EPA-600/9-85-020a, Paper No. 13 (July 1985), compare the effectiveness of three sulfur sorbents on calcium utilization (averaged for Ca/S molar ratios of 1 and 2, between injection of the sorbent through the burner and downstream injection of the sorbent at temperatures of about 2200.degree. F. to 2300.degree. F. For downstream injection Rakes, et al. found a slight increase in calcium utilization for limestone, and a marked increase in calcium utilization for calcium hydroxide and calcium dihydrate.
Kelly, et al., Pilot-Scale Characterization of A Dry Calcium-Based Sorbent SO.sub.2 Control Technique Combined With A Low-NO.sub.x Tangentially Fired System, Proceedings: First Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies, EPA-600/9-85-020a, Paper No. 14 (July 1985), investigated the effectiveness of sulfur sorbents when injected in the combustion zone and in downstream locations. Kelly, et al. concluded that sulfur sorbents should be injected downstream to avoid sorbent deactivation by high peak temperatures in the combustion zone. Kelly, et al. also suggest that the residence time of calcium-based sulfur sorbents in the temperature zone between about 2250.degree. F. to about 1800.degree. F. should be maximized to maximize sulfur capture.
Overmoe, et al., Boiler Simulator Studies On Sorbent Utilization for SO.sub.2 Control, Proceedings: First Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies, EPA-600/9-85-020a, Paper No. 15 (July 1985), compared the sulfur capture between downstream injection and fuel injection for limestone and dolomite sorbents. The results of both sets of tests suggest that downstream injection of sorbents increases the sulfur capture capacity of the oorbents. The sorbents were injected downstream at temperatures of about 2250.degree. F.
The United States Environmental Protection Agency has been conducting a Limestone Injection Multi-Stage Burner (LIMB) Program for research on methods for reducing sulfur oxides emissions from the combustion of coal with limestone sulfur sorbents. The primary emphasis of this multi-million dollar LIMB Program has been toward injection of limestone sulfur sorbents downstream from the combustion zone where temperatures have cooled to about 2250.degree. F. In such systems, the limestone sorbent must be rapidly and completely dispersed throughout the cross-section of a boiler where the combustion gases are rapidly flowing and the area of the cross-section is typically about 2500 square feet.
Magnesium compounds do not capture sulfur compounds to any appreciable extent at the high temperatures found in a boiler environment. Above 1500.degree. F. and for gas concentrations typically found in the boiler, magnesium sulfate is unstable. Magnesium oxide, however, is produced in the high temperature oxidizing environment of the boiler. Below 1500.degree. the reaction of sulfur dioxide with magnesium oxide is exceedingly slow, while sulfur trioxide readily reacts with magnesium oxide below 1500.degree. F. The concentration of sulfur trioxide in the boiler gases is quite low, however, and its formation from the reaction of sulfur dioxide with oxygen is very slow unless catalyzed.
While various methods for reduction of sulfur oxides and nitrogen oxides emissions are known, such methods are expensive and/or not effective. Wet-scrubbing is the principal commercial method for sulfur oxides reduction. While this technology effectively removes sulfur oxides, it is expensive and can add from thirty-three to sixty-five dollars per ton to the cost of coal to achieve a ninety percent reduction in sulfur emissions. The cost to retrofit an existing facility with a wet scrubber can, in some cases, equal the cost of the facility. The cost of such retrofits requires recapitalization. For older facilities, amortizing such costs over a short remaining lifespan is impractical.
Although the current Federal Clean Air Act New Source Performance Standards requiring a ninety percent reduction in pounds of sulfur per million Btu only apply to facilities built after 1977, environmental concerns about acid rain could initiate new legislation applying to older facilities, as well. Additionally, some states have stricter laws than current federal legislation. Presently, most coal fired utility boilers are more than twenty-five years old and have no desulfurization equipment. As discussed above, major disadvantages are associated with retrofitting such facilities with wet scrubbing equipment.
Accordingly, there is a need for a method for economically reducing emissions of sulfur oxides in existing coal-burning facilities. The present invention involves a customized fuel composition for reduction of sulfur oxides. Eighty percent reduction of sulfur oxides can be achieved with the present composition and still be more economical than wet scrubbing processes. Additionally, while the composition is more expensive than untreated coal, any cost increases to electric utilities can be incorporated into existing rate bases without the need for recapitalization. The present fuel composition is also advantageous because a utility can switch to use of the composition witout a need to change existing storage facilities.
The use of various sulfur sorbents and sulfation promoters is known, as is the use of coal which has been cleaned to reduce inorganic sulfur. It has not, however, been previously recognized that the combination of a refined coal having substantial reductions in pyrite and other ash-forming minerals, a sulfur sorbent including a calcium and a magnesium component, a sulfation promoter, and a catalyst for the conversion of sulfur dioxide to sulfur trioxide, when combusted in an oxygen restricted combustion zone, can achieve highly effective sulfur oxides and nitrogen oxides reduction.
Formation of sulfur oxides is reduced by the present invention because of the low pyrite content in the refined coal. Additionally, the refined coal is low in silicates and aluminosilicates, which otherwise effectively compete with sulfur oxides for reaction with sorbents at higher temperatures. Sulfur oxides which are formed from sulfur in the coal react with calcium and magnesium components of the sorbent. When magnesium is present in the sorbent in dolomitic form, the rate of calcium sulfation at higher temperatures is increased although the magnesium portion of dolomite is not sulfated. The use of a catalyst for production of sulfur trioxides enhances sulfation by the dolomitic magnesium which remains unsulfated by assuring that sufficient quantities of sulfur trioxides are present. Sorbent sintering and formation of nitrogen oxides are reduced by lower flame temperatures which are achieved by use of a low NO.sub.x burner and the endothermic conversion of sorbents to the oxide form.
In addition to reducing sulfur and nitrogen oxides, the fuel composition of the present invention has a number of favorable operational impacts on a boiler. The cost of pulverizing coal is reduced because less power is required to break up an agglomerated material than coal. Slagging is reduced because the refined coal has low amounts of ferrous iron and silicates. Fouling is reduced because of the low sulfur content of the fuel and the addition of calcium. Ash burden is decreased because, although the addition of sorbents increases the ash, a low ash coal is the starting material for the fuel composition.